Electron discharge device



Dec. '17, 1940. A. v. HAEFF ETAL ELECTRON DISCHARGE DEVICE Filed Sept. 13, 1939 4 Sheets-Sheet l INVENTORS Wm m EM 0 s w H A MP. ww 0 M N A 4 Sheets-Sheet 2 Filed Sept. 13, 1939 5w mum w M M m E 0 W T. 1 n wm E0 m N A Y B 33E \JR 9? m NYM V M U? NWJ H u- W IF N 1 J1)? (Q E w? 3w Q \w @w ww. mw l M Q Q Dec. 17, 1940. A. V. HAEFF EI'AL 2,225,447

ELECTRON DISCHARGE DEVICE I Filed Sept. 13, 1939 4 Sheets-Sheet 4 NV EN TORS HAEFF AND I ANDRE W V.

BY 0Y0 RSMITH WW ATTORNEY.

- Patented Dec. 17,1940

PATENT "OFF-ICE ELECTRON mscnanoe navrcn Andrew V- HaefiL'East Orange, N. J., and Lloyd P. Smith, Ithaca, N. Y., assignors to Radio Corporation of America, a corporation Delaware Application September 13, 193e, Serial "No. 294,612 13 Claims. (01.250-161) Our invention relates electron discharge devicesgmoreparticularly to such devices employing' a beam of electrons and suitable for use at I high frequencies.

lntd'evices employing electron beams it is es-,

sential that the beams be prevented from diverging. The principal causes of divergence in electron beams are (1) the mutual repulsion of the electrons themselves, (2) the defocusing of the static electric fields, and (3) the defocusing produced by electric fields which vary with time. The latter is particularly objectionable in devices where electrons are required to traverse a gap across which there is a potential difference which changes appreciably during the time 01! electron transit. This phenomena will be discussed in greater detail below. Eflorts have been made to prevent or neutralize divergence by means oft electrostatic fields imposed uponthe electrode system but in many applications this isundesirable from a practical point of view because of the necessity of introducing additional electrodes and other cohsequent circuit complications. While the defocusing effects may be compensated for by the use oi'a suificiently inj tense. uniform magnetic field in the direction of the beam,'-it is sometimes diillcult to produce the intense. uniform magnetic field throughout the volume occupied by the beam, particularly in those cases where circuit considerations do not permit the source of the magnetic field, that is the solenoid or permanent magnet. to be located in the immediate-vicinity of the beam.

It isthe principal object, of our invention to the beam or by static or varying electric fields is a substantially neutralized,

f The novel, features which we believe to be characteristic of our invention are set forth with particularity in th appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawings in which Figurel is. a diagram showing the phenomena which are neutralized by our invention, Figures 2 and 3 are diagrams illustrating the fundamental principles involved in our invention, Figures 4 to 8 inclusive-are diagrams explaining the operation of one form of an electron discharge device-to which our invention is particularly appartially diagrammatic views of. electron discharge devices made according to our invention andv their associated circuits. 'lo illustrate .the action of electric fields whic 'plicable, and Figures 9 to 12 are cross section result in defocusing of an electron beam when.

the beam passes a gap between two adjacent electrodes at diil'erent potentials, reference is had to Figure 1. vIn Figure 1 tubular electrodes Ill and II are separated by a gap as indicated and an accelerating field for the electrons is provided by maintaining electrode ll at'the higher potential. The e ectrostatic lines of force are indicated by the dotted lines l2 and the equipotential lines of force by the thin full lines .13. Electrons traveling along the parallel paths M as indicated in the left hand side of the figure are accelerated toward the right hand side. When an electron is traveling from left to right and .the potential difference is such as to accelerate the electron, the electron first encounters a radial component'of electric force towards the'cenindicated in Figure 1. After crossingthe center 'of the gap the electrons are acted upon bya radial force in an opposite direction tending to remove the electrons from the beam. Under static conditionsthe net effect of the radial components of force is to produce convergence because the accelerated electrons will traverse .the diverging part of the field in a shorter time than the converging part. That is, the electrons will be subjected to -a converging fieldon' the left side 'of the gap for a longer time than to a diverging field of the same strength on the right side.

However, if, the field increases appreciably during the electron transit time, there may be a divergence of the beam because although the period of dwell within the field on the right hand side is shorter, the electrons experience a greater outward force, during their transit on the right hand side a'scompared with the force toward the center while traversing the field on'the left hand side of the gap. On the other hand, it the field is decreasing during transit time, the net result will be to converge the beam more sharply than when a static field is employed. That is, although the electron will dwell within the field at thev right hand side of the gap for a longer period of time than atthe left hand side, the forces on the right hand side are considerably weaker and thus ,the beam will remain focused.

If the electrons enter a retarding field, that is with the electrode I0 at a higher potential under static conditions, the electrons remain for a longer period of time on the right hand side of the gap than on the left hand side. A divergin effect is produced on the left side but the beam will converge since converging forces on the right side of the gap act for a longer period of time. If the field is being increased, that is the left hand electrode increases in potential, convergence will be even greater. On the other hand if the field is decreasing with the potential on the electrode in decreasing and the potential on the electrode ll increasing in a. positive direction, if the change is rapid enough divergence may result.

The various defocusing actions described above can be avoided by the use of our invention usin a system of magnetic lenses. The defocusing due to the variations of the electric field with time is compensated most effectively when the magnetic field of the lens occupies the same region as the defocusing electric field.

The basic principle of our invention is illustrated in Figure 2. Surrounding the tubular electrodes l0 and II are tubular members 18 and it of ferromagnetic material, preferably iron, similar in shape to the conductors producing the electric field. That is, there is provided between these electron lens members a gap registering with the gap between the tubular electrodes. The distribution of the magnetic field is similar to that of the electric field. The magnetic field is chosen sufliciently large so that convergence of the beam is produced irrespective of the diverging effect of a rapidly changing electric field. In this figure the electric field is represented by the dotted lines and the magneticfield. by the solid lines. Themagnetic field of course remains constant although the electric field may change.

In such a lens the force on the electrons producing convergence depends on the square of the magnetic field intensity. As is shown in Figure 2 the magnetic field increases as the beam boundary is approached. Therefore, the outermost electrons will receive the greater focusing action so that all the electrons passing through the .lens'will be converged. For this reason 'amagnetlcle'ns of this type has a very important advantage over the use of a uniform magnetic field. In addition the localization of the magnetic field in the regions where it is most needed makes the energy required to establish the requisite focusing field considerably less than for the uniform field. The same type of mag-' netic lens can be effectively used to prevent divrgence from any cause such as space charge and defocusing eflects of electrostatic fields.

'O'ne of the most important. uses of magnetic le'nse's'described is in connection withelectron beam tubes designed for operation at high, frequencies. However, the use of the lens of the form illustrated in Figure 2, where high frequency electric fields are ,efi'ective, would involve large energy loss in the ferromagnetic material due to currents induced by high frequency fields in which the electrodes would be immersed during' operation of the tube. In order to avoid this -in accordance with our invention we coat or of .0001 inch.

Magnetic lenses of the type described above have been successfully used for focusing an electron beam in the high frequency inductive output tubes of the type described and claimed in a co-pending application, Serial No. 254,239 filed February 2, 1939, in the name of Andrew V.

.Haeif, one of the co-inventors' of the present application, and assignedto the same assignee as the present application. In order that the construction and the operation of such a tube may be understood, reference is had to Figures 4 to 8 inclusive.

In Figure 4- is schematically shown the longitudinal schematic section of a quarter wave concentric line tank circuit comprising an inner tue bularconductor 20 which may be cylindrical in cross section, and a hollow outer tubular conductor 2| concentric with the inner conductor 20 and electrically connected to the inner conductor 20 by the conducting plate 22. A second tubular conductor 24 which may be referred to as the aperture extension is coaxiaLwith the conductor IQ and spaced axially from the conductor 20 to provide a gap 25. This tubular conductor 24 and the outer conductor 2| are connected by the conducting plate 23. This arrangement provides a quarter. wave concentric tank circuit. If a negatively charged body 20 is projected axially through the inner-conductor 20 from left to right, the conditions of the charge distribution on the circuit as the body 26 is moved along the interior of conductors 20 and 24 is indicated in Figures 4 to 6 inclusive. As shown in the figures, there is a positive charge equal to the negative charge induced on the inside of the inner conductor near the body. However, initially no charge appears on the outer surface of the inner conductor 20. The

induced charge moves with the charged body along the inner surface of conductor 20 until the end of the inner conductor 20 is reached.

During thepassage of the charged body across the gap 25, the charge is partially imaged on the end of the inner conductor 2. and partially on the outer conductor 24 as shown in Figure 5. Thepassage of the charged body beyond the gap 25 into the conductor 24 causes the induced charge all to appear on the inner surface of the conductor 2| as shown in Figure 6. The in-.

duced charge in transferring from the end of the inner conductorto the conductor 24 flows back over the outer surface of the inner conductor 20 and the inner surface of the conductor Jected past the gap in proper phase and freh quency relationship with respect to the resonant frequency. of the tank circuit, the circuit may be made to oscillate vigorously merely by the passage of the charged bodies past the gap.

Figure 7 illustrates the configuration of the electric and magnetic fields within the resonant space of the tank circuit when the latter is excited. The solid lines llrepresent the electric field distribution and the circles 28 represent the magnetic lines of force. The dashed lines 29 represent the equipotential surfaces in the gap. Along the major part of the length of the tank circuit the direction of the electric field is substantially radial. However, at the gap the electric field has an axial component. The electric field does not penetrate very far inside the open end of the inner conducting member 20 or inside the conductor 24, but is confined effectively to the space defined approximately by the limiting equipotential lines 29 shown in thefigures. and inside the conductor 24 is essential field free, therefore no work will be done on a charge moving inside the inner conductor '20 by the electric field until the charge reaches the gap 25. If.

the charge traverses the gap at the instant when the electric force is in the direction from Him 24, the charge will be decelerated, its energy being given up to the tank circuit. A charge crossing the gap during the opposite half cycle when the field is reversed will be accelerated and absorb energy from the circuit. If, however, the number of charges traversing the gap during the first half cycle is greater than during the second, the net eifect will be that energy is supplied to the tank circuit. d

Thus, the tank circuit may be excited by passing groups of electrons at the proper frequency across the gap between-the conductors 20 and 24. The motion of the electrons in the interior of the inner conductor 20 has no effect on the current in the tank circuit. Also high frequency electromagnetic fields, which will .be generated within the resonating space of the tank circuit, penetrate but a short distance inside the conductor 20 and conductor 24 which act as a screen electrode so that the electrons will be influenced by these fields only during their passage across .the gap.

In Figure 8 is shown schematically in section an electrode arrangement ofa tube embodying and operating on the principle described above.

5 Mounted within the inner conductor 20 is a convventional cathode 30 and a'grid 3|, which supply 'ode will fiow toward the collector.

the pulses of electrons in'the proper phase relation necessary to excite the tank circuit. A collector electrode 32 may be placed beyond the screening electrode or aperture extension 24'.

If 'now a high potential is applied between the cathode and the tank circuit including electrodes 20 and 24 and also between the collector 32 and cathode 30, a stream of electrons from the oathfrequency voltage is applied between the control gridand the cathode the electron stream will be periodically modulated in intensity. Pulses of electrons traversing the gap 25 will induce high frequency currents between the electrodes 20 and 24'. If the excitation frequency is adjusted to the resonant frequency of the tank circuit a. high impedance will exist across the gap 25 at this frequency. The induced currents, therefore.- 1

Q will produce a high radio frequency voltage across the gap 25. The phase of this voltage at or near resonance will be such as to-decelerate electrons traversing,,,the gap during the half period of maidmiim'intens'ity of the electron current in the stream.

The energy lost by the electrons is transformed by the tank circuit into the energy of the electromagnetic field within the resonating space between the inner and outer conductors and then 7 may be conveyed to the useful load by means of The space inside .the inner conductor 20' If a high I a coupling loop such as, for example, 33 extending through an aperture in the outer tubular conductor 2| of the tank circuit.

The high frequency electromagnetic field ex' isting in the resonant space of the tank circuit penetrates only a short distance inside the to.- bular electrode 20 and inside the tubular screen electrode 24. Therefore, by positioning the control electrode 3| at a suitable distance from the gap 25 the coupling between the input electrodes 30 and 3| and the output electrodes 20 and 24' cause the functioning of the tube does not depend critically upon the electron transit time. This is because the electrons are efiective in exciting the output circuit only during the short periodof time that they pass through the field' extending through the gap 25. The current collecting electrode 32 can be operated at a much lower potential than the conductors 20 and 24 and in order to obtain a high emciency it is usually operated at a potential just sufiicient to collect all decelerated electrons. To improve the functioning of the deivce an electrostatic or magnetic focusing of the electron stream can be utilized to prevent electrons from impinging on the high potential electrodes 20 or 24. Thus these electrodes will not dissipate energy and all of the power generatedin the tube will be supplied by the low voltage collector power supply.

A beam tube of the type described and embodying our present invention is shown in Figure 9. The evacuated envelope 40 contains within it the cathode 4| of' the indirectly heated type, a control grid 42 and a collector electrode 43, and positioned between these electrodes the accelerating and focusing electrodes 44 and 45. A beam of electrons is directed from the cathode to the collector electrode'43. In the construction shownthe concentric line tank circuit comprises the inner tubular members 46 and 41 spaced axially to provide a gap between their ends. The outer concentric tubular member 48 is electrically connected to the inner tubular members by the end discs or plates 49 and 50. The gap between the two inner coaxial cylinders is positioned between the electrodes 44 and 45.

In accordance with our invention the magnetic circuit of ferromagnetic material includes the tubular members 5|, 52 and 53 positioned toprovide two gaps 54 and 55, one of which registers with the accelerating electrode 44 and the other of which registers with the gap between the coaxial inner tubular members. In this way two lenses are formed. The tubular members of magnetic material forming the lens system are encased by means of highly conductive material 46' and 41' electrically connected to and forming part ofthe inner tubular coaxial members so that 0 the ferromagnetic material is completely shielded from the high frequency fields. The electron beam which originates at cathode 4i is.accelerated and focused by the control grid 42 and accelerat ing electrode 44, being finally collected at the col-- lecting electrode 43. The lens formed by members ti and 52 prevents divergence oi the beam caused by electrodes 62 and M. The lens formed by the gap between tubular magnetic members 52 and 53 compensates for the divergence caused by the electric field across the gap 55. By adjusting the length of the gaps in the magnetic circuit and adjusting the current in the magnetic coil 59, the core 58 of which forms a U-shaped magnet with the legs 51 and 51', it is possible to w prevent electron bombardment of electrodes 44 and 45 as well as the, glass walls of the tube. The power required to do this is a small fraction of that needed when a uniform field is used.

The circuit arrangements and voltage sources 15 are also disclosed. The leads 6i and 62 connected to the source of voltage 60 provide'heating current to the heater (not shown) of cathode 4!. Lead 62 connected to cathode 4| and lead connected to the control electrode 42 forms with the 20 bridging member 64 a Lecher wire system. The input circuit is connected to a driver by means of the loop 55. The tank circuit is maintained at a positive potential by means of the conductor 66 which is electrically connected to the source of as supply 61. Voltage is supplied to the collector from voltage source 58, the collector being maintained at a lower potential than the tank circuit. The accelerating electrode 45 is connected electrically by conductor $5 to the positive side of St) voltage source 61. The load is connected to the tank circuit by means of the loop 69 extending within and coupled to the outer tubular member 48. I i

In Figure is shown a modification of an as electron discharge device made according to our invention in which the separate magnetic lenses are independently controlled by their individual magnetic circuits. Here the cathode 4| and control electrode d2 provide the modulated beam,

0 which is collected by collector electrode 43'. The tubular members 46' and 41' with the outer tubular member 48' and the connecting side members 49' and 50' form the tank circuit, The magnetic lens tubular members 51' and 52' are separated 45 for providing a gap registering with the gap between tubular members 46' and 41'. These electromagnetic members are coupled to the coil 59' by means of legs 12. Thus the intensity of the magnetic field across the gap can be controlled so by the coil 59'. The tubular electrode 46' has adjacent and surrounding it the magnetic tubular member 14 coupled magnetically with the coil 16 by leg 15. Thus the magnetic lens within the electrode 46' and adjacent the cathode and grid can 55 be regulated independently or the magnetic lens at the gap 13 to provide the focusing desired. These magnets could of course be permanent magnets instead of magnets and 16. Within the collector electrode is mounted a rod 10 maintained at cathode potential for causing the electron beam to be spread and to prevent -secondaries from leaving the collector electrode and going back to the tubular member 41'. The shield 1| prevents electrons from leaving the interior of as the collector 43' through the aperture through which the rod 10 extends. This feature is covered in the co-pending application of Andrew V. Haefl,

Serial No. 292,812, filed August 31, 1939, and assigned to the same assignee as the present ap 70 plication.

In Figure 11 is shown an arrangement whereby the output electrical circuit and magnetic lens are entirely contained within the glass envelope of the tube. The grid, cathode and collector are 75 arranged in the manner similar to that in the assess? magnet 59' to pass through the glass walls at d and at e, the overlapping areas d and e 01 internal and external magnetic material being made large enough to keep the reluctance through the glass relatively low. As in other forms a-mal5 terial of high electric conductivity is used on the surface of magnetic material to prevent high frequency losses.

In another form of our invention in Figure 12 we show the application of our invention-to an electron discharge device utilizing a plurality of beams focused by a system of lenses formed by the grid-like structure 01' copper coated magnetic material as shown. Each of the beams is approximately rectangular in cross section, originating from the rectangular cathodes 80 controlled by control electrodes 8'! and passing through the reduced grid-like portions 85' and. 86' of the members 85 and 86 and collected'by collector electrode 82 provided with rectangular shaped pockets 83 registering with the beams. The magnetic circuit for energizing the lens is common to all the lenses and coincides with the discs 96 and 9| forming with the cylindrical part 89 and the inner tubular members the electrical as output circuit. The part 89 of the electrical circuit is-made of non-magnetic material so that the magnetic circuit is completed through the permanent magnets 92 and 93 in contact with the discs 81 and 90, the portions of magnetic ma- 0 terial 90 and 9! completing the magnetic circuit with the permanent magnets.

The application of our invention to tubes used in actual practice has vastly improved the operation of these tubes at high frequencies. and results in a more efilcient tube having stable characteristics over a wide range of frequencies.- The defocusing action of the high frequency fields is substantially eliminated and bombardment of electrodes in the glass envelope by the beam is practically eliminated.

While we have indicated the preferred embodiments-oi our invention of which we are now aware and have also indicated specific applications for which our. invention may be employed, it will be'apparent that our invention is by'no means limited to'thejexact forms illustrated-or the use indicated, butthat many variations may b made in the particular structure. used and the purpose for which it is employed without departing from the scope of our invention as set forth in the appended claims. 7

What we claim as new is: 1. An electron discharge devicehaving a pair of conducting members separated by a gap, a conducting member connected to said pairoi conducting members for forming an oscillating tank circuit, means for projecting a stream of electrons across said gap, magneticmans adjacent said conducting members and having a 7 gap registering with the gap between the conducting members for providing a. magnetic lens at said gap, means for modulating saidstream of electrons prior to its passage across said gap and a collecting electrode for collecting electrons v or c o-axial tubular electrodes spaced axially to '5 provide a gap between the tubular electrodes,

tubular'members or magnetic material closely adjacent said axially spaced tubular electrodes and provided with a gap registering with the gap between said axially spwed tubular electrodes to provide a magnetic lens at said gap, means for projecting electrons axially oi. said coaxial tubular electrodes across said gap, means for collecting electrons passing through said tubular electrodes, and a sheath of non-magnetic material or high electrical conductivity around said tubular members of magnetic material.

3. An electron discharge device having a pair of coaxial tubular electrodes spaced axially to provide a gap between the tubular electrodes, tu-

bular members or magnetic material closely adjacent said axially spaced tubular electrodes and provided with a gap registering with the gap between said axially spaced tubular electrodes to provide a magnetic lens at said gap, means for Q5 projecting electrons axially or said coaxial tubular electrodes across said gap and means for collecting electrons passing through said tubular members, and a solenoid magnetically coupled to said tubular members of magnetic material.

30 4. An electron discharge device having a oath-- ode for supplying electrons, a collector electrode spaced from said cathode for receiving said elec- I trons, means including a pair of coaxial tubular members spaced axially to provide a gap be- 35 tween said tubular members, said tubular members being so positioned-between the cathode and collector electrode so that electrons moving from the cathode to the collector electrode will pass axially through the tubular members across said between the axially spaced tubular members to provide a magnetic lens at said gap.

5. An electron discharge device including a quarter wave concentric line output tank circuit 50 having a pair of coaxial tubularmembers spaced axially to form a gap and a concentric outer tubular member connected by a conducting plate at each end to one of said tubular members, a cathode and grid positioned adjacent the end or one 5 or said pair or coaxial tubular members for supplying electrons axially of said coaxial tubular members across said gap and a collecting electrode adjacent the end of the other coaxial tubular member for receiving the electrons from 60 the cathode, means including tubular members or magnetic material spaced. to provide a gap registering with the gap between the coaxial tubular members for providing a'magnetic lens at said gap, the electrons projected through said coaxial members from said cathode 'to said collecting electrode being iocussed into a well-defined beam-by said magnetic lens.

6. An electron discharge 70 collecting electrode for receiving the beam of electrons, a pair or coaxial tubular electrodes axially spaced to provide a gap-and surround ing-the discharge path between the. cathode and anode whereby electrons-from the cathode to 75 the anode traverse the gap between said pair at said gap.

device including means lorprovidlng a beam of electrons and a I of coaxial tubular electrodes and an accelerating electrode surrounding the path of the beam between the cathodeand the gap, tubular members of magnetic material positioned closely adjacent the tubular electrodesand provided with 5 gaps registering with the accelerating electrode and with the gap between the coaxial tubular electrodes to provide a pair of electron lenses for said accelerating electrode and at said gap.

'7. An electron discharge device having an en- 10 -ve1ope containing a cathode and grid for providing a source of modulated electrons,- and.

' means for forming said modulated electrons into a beam, a collector electrode for receiving said electrons, and-a quarter wave concentric line 15 tank circuit having a pair of coaxial tubular I membersspaced axially to provide a gap and surrounding said envelope and the discharge path between the cathode and collector electrode, and positioned so that the gap between 20' the tubular members is intermediate the cathode and collector electrode, and magnetic means adjacent said coaxial tubular members and having a gap registering with the gap between the conducting members for providing a magnetic lens 8. An electron discharge device having an envelope containing a cathode and grid for providing a modulated stream of electrons, a collector electrode for receiving said electrons, and a quarter wave concentric line tank circuithavihg a pair of coaxial tubular members spaced axially to provide a gap surrounding said envelope and the discharge path between the cathode and collector electrode, and positioned so that the gap between the tubular members is intermediate Y the cathode and collector electrode, an accelerating electrode adjacent said grid and tubular members of magnetic material adjacent and. coaxial with said coaxial tubular members and having a gap registering with the gap between the coaxial tubular members and a gap adjacent the accelerating electrode for providing a mag-' netic lens at said' gap and for said accelerating electrode.

9. An electron discharge device provided-with means for providing a projected beam of electrons, a collector electrode for receiving said electrons, a pair of tubular coaxial electrodes spaced axially to provide a gap and surrounding .50 the discharge path of said beam and positioned between said electron beam supplying means and the collector electrode, and tubular members of magnetic material adjacent thetubular electrodes and surrounding the discharge path and having a gap registering with the gap between the tubular electrodes and a gap adjacent the beam supplying means whereby a pair of electron lenses is provided for maintaining the beam of electrons focused during operation of the co tube.

10; An electron discharge device provided with means for providing a projected beam of electrons, a collector electrode for receiving said electrons, a pair of tubular coaxial electrodes spaced axially to provide a gap and surrounding the discharge path of said beam and positioned between said electron beam supplying means and the collector electrode, and tubular members of magnetic material surrounding the discharge to path adjacent the tubular electrodes and having a gap registering with the gap between the tubular electrodes and a gap adjacent the beam supplying means whereby a pairi of electron lenses is provided for maintaining a beam of 7 electrons focused during operation of the tube, and solenoid means operably associated. with the magnetic tubular member for individually controlling the magnetic field produced at each gap between the tubular members'of magnetic material.

11. An electron discharge device including a cathode and a grid for providing a modulated stream of electrons, a collector for receiving said electrons a quarter wave concentric line tank circuit'having a pair of coaxial tubular electrodes spaced axially to provide a gap and sur- 12. An electron discharge device having a pin-- rality of beam supplying means for supplying a plurality of parallel beams and means for 001- lectingsaid beams, tubular electrodes provided with a plurality of apertures registering with said beams and spaced to provide a gap through which said beams are projected, magnetic means surrounding said apertures and having a gap registering with the gap between the tubular members, extensions from said magnetic means and permanent U-shaped magnets cooperating with said extensions to provide a magnetic ctr cuit whereby a magnetic field is produced at said gaps to provide a magnetic lens for said beams of electrons.

13. An electron discharge device having an envelope containing a cathode and a grid for providing a source of modulated electrons, a collector for receiving said electrons and a quar ter wave concentric line tank circuit having a pair of coaxial tubular members spaced axially to provide a gap and surrounding the discharge path between the cathode and collector electrode and positioned so that the p between the ular electrodes is intermediate the cathode and collector electrode, -magnetic means adjacent and coaxial with the coaxial tubular electrodes and having a gap registering with the 8. 9 between the tubular electrodes for providing a magnetic/lens at saidzgap, and radial extensions connected to said magnetic means extending to the interior surface or the envelope, and a permanent electromagnet of tubular shape surrounding the exterior-oi said envelope and posi tioned to complete a magnetic circuit through said extensions on said magnetic means.

ANDREW v. m xer. LLOYD P. smu 

